1. Field of the Invention
This invention relates to a cable with recyclable covering.
In particular, the invention relates to a cable for transporting or distributing medium or high voltage electricity, comprising a layer of recyclable thermoplastic polymer covering with superior mechanical and electrical properties, enabling it, in particular to be used for high operating temperatures and for transporting electricity at high power.
2. Description of the Related Art
The requirement for products of considerable environmental compatibility, composed of materials which, in addition to not damaging the environment during production or utilization, can be easily recycled at the end of their life, is now fully accepted in the field of electrical and telecommunications cables.
However the use or materials compatible with the environment is conditioned by the need to limit costs while, for the more common uses, providing a performance equal to or better than that of conventional materials.
In the case of cables for transporting medium and high voltage electricity, the various coverings surrounding the conductor commonly consist of polyolefin-based crosslinked polymer, in particular crosslinked polyethylene (XLPE), or elastomeric ethylene/propylene (EPR) or ethylene/propylene/diene (EPDM) copolymers, also crosslinked. The crosslinking, effected during extrusion, gives the material satisfactory performance even under hot conditions during continuous use and with current overload.
It is well known however that crosslinked materials cannot be recycled, so that manufacturing scrap and the covering material of cables which have reached the end of their life can be disposed of only by incineration.
Moreover in some cases the external protection sheath of the cable is of polyvinylchloride (PVC), which if using conventional methods (for example by density difference in water) is difficult to separate from the crosslinked insulating material, in particular from crosslinked polyolefins containing mineral fillers (for example from ethylene/propylene rubber), neither can it be incinerated because combustion produces highly toxic chlorinated products.
There is therefore a need in the field of medium and high voltage electricity transport cables for insulating coverings consisting of recyclable polymers which have good electrical and mechanical properties.
Of uncrosslinked polymers, it s known to use high density polyethylene (HDPE) for covering high voltage cables. HDPE has however the drawback of a lower temperature than XLPE, both to current overload and during operation.
Thermoplastic low density polyethylene (LDPE) insulating coverings are also used in medium and high voltage cables. Again in this case, these coverings are limited by too low operating temperature (about 70° C.).
Another material potentially suitable for cable production is polypropylene (PP). In common use this term is used to indicate high crystalline isotactic PP, a thermoplastic material of high mechanical performance. In reality, isotactic PP cannot be used as a cable covering material, mainly because of its high rigidity, so that the attention of cable manufacturers has turned to other materials based on PP but possessing good flexibility (the so-called “flexible PPs”).
For example, patent application WO 96/23311 describes a low voltage, high current cable in which the insulating covering, the inner sheath and the outer sheath are of the same uncrosslinked polymer, coloured black by the addition of carbon black. The use of the same material means that no separation of said components is required for recycling. For a maximum working temperature of 90° C. it is stated that heterophase thermoplastic elastomers can be used consisting of a polypropylene matrix within which an elastomeric phase of EPR or EPDM copolymers is dispersed.
Patent applications EP-A-475,306 and EP-A-475,307 describe a substantially amorphous elastomeric polypropylene homopolymer having a melting point between 145° C. and 165° C. and a heat of fusion between 4 and 10 cal/g and comprising a diethyl ether soluble fraction between 35 and 55%, this fraction having a relative viscosity of less than 1.0 dl/g and substantially no isotactic crystallinity. This polymer is produced by homopolymerization of propylene in the presence of a Ziegler-Natta catalytic system without electrondonors, comprising a solid catalyst based on titanium tetrahalide and aluminium trihalide supported on magnesium chloride, with aluminium trialkyl as co-catalyst. A potential use of the amorphous polymer so obtained is suggested for producing films.
Patent application EP-A-527,589 describes a polymer composition comprising: a) 20-80 wt % of an amorphous polyolefin comprising propylene and/or 1-butene in a quantity of at least 50 wt %, and b) 20-80 wt % of crystalline polypropylene. The composition is prepared by mechanically mixing amorphous polyolefin with the crystalline polypropylene. This composition is said to have excellent flexibility under cold conditions while maintaining the high hot mechanical strength typical of polypropylene, and hence suitable as an insulating material for cables.
The Applicant believes that the solutions already proposed for insulating medium or high voltage electric cables with a recyclable polymer are unsatisfactory. In particular, those polypropylene-based materials indicated in the prior art are unable to combine a mechanical performance which is satisfactory under both cold and hot conditions (in particular good mechanical strength and sufficient flexibility) with considerable electrical reliability.
In particular, heterophase materials such as the heterophase thermoplastic elastomers suggested in WO 96/23311 in which an elastomeric EPR or EPDM phase is dispersed in domains of the order of a few microns within a polypropylene matrix, are characterised by microscopic dishomogeneity, which can induce the formation or cavities at the interface between the elastomeric phase and the thermoplastic phase. With the passage of time and in the presence of an electrical field, these cavities can result in degradation of the material and hence perforation of the insulating layer.
The Applicant also believes that the amorphous polypropylenes, such as those described in EP-A-475,306 and EP-A-473,307, cannot satisfactorily be used for electric cable insulation. In this respect, as these materials have a high amorphous phase content for a low molecular weight, as indicated by the presence of a diethyl ether soluble fraction between 35 and 55 wt %, they show poor mechanical strength, in particular under hot conditions.
Again, the present applicant has found that granules produced by mechanically mixing amorphous polypropylene with isotactic polypropylene, as described for example in EP-A-527,589, show an oily surface and considerable stickiness on storage, clearly indicating partial insolubility between the two polymers with migration of the low molecular weight fractions towards the material surface. This problem results in numerous material processability problems, as the granules tend to pack together making it difficult, for example to feed the granules into an extruder. Moreover, in the finished article the presence of an oily low molecular weight product on the surface of the insulating layer can cause poor adhesion between the insulation and the semiconductive layers, with possible separation during cable operation and consequent partial discharges.